The present disclosure relates generally to an improved design for assembling a fuel cell stack, as well as to distribute an acceleration load over a fuel cell stack to secure and maintain the relative position of the fuel cells within the stack after exposure to impacts and other high acceleration loads.
A significant benefit to using fuel cells to convert a fuel into usable electricity via electrochemical reaction is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected in series along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
The delivery of the reactants to the MEA—as well as the removal of the byproduct water and the delivery of the cell-generated electrical current to the load—is facilitated through a gas-permeable diffusion medium (also called a gas diffusion medium (GDM)) and a bipolar plate the latter of which is sized to be placed about the periphery of a corresponding fuel cell. One way to improve alignment of each plate and cell assembly during stack formation is to include one or more datum structures (also called datum) that are built in, secured to or otherwise cooperative with one or more of the bipolar plates. In one common form, the datum may accept a datum pin to promote such alignment in the stacking (for example, Y-axis in a Cartesian coordinate system) dimension.
Fuel cell stacks placed within vehicles must be able to withstand severe load changes from acceleration and deceleration of the vehicle, as well as from crashes, accidents and related impacts. In particular, in order to continue to perform after exposure to high acceleration loads (for example, up to 160 g or more) during disruptive events such as a vehicle crash, the position of the fuel cells that make up the stack must be retained relative to one another. In the event of high acceleration, deceleration, or impact of the vehicle, a high shearing force may cause sliding between adjacent cells of the stack (especially in the X-Z plane of the aforementioned Cartesian coordinate system). Small displacements between cells can result in large cell block displacements when all cells are considered (e.g. a 100 micron cell shift can result in a 30 mm cell block shift for a 300 cell stack assembly). Such problems may be exacerbated by cold start conditions where thermally-induced contraction may reduce the Y-axis compressive retention load that was placed on the cells during stack assembly, as well as by reduced inter-cell friction brought about by the use of surface treatments or inserts that may have low coefficient of friction attributes. The use of adhesives or supplemental support structure may tend to meliorate this problem to some degree, but in the process tends to add weight and complexity and—in the case of adhesives—is not conducive to subsequent stack disassembly for repair or diagnostic analysis.